Optical fiber having core segment with refractive-index grating

ABSTRACT

A grating is induced in the core of a hydrogen-loaded high-germanium-content optical fiber using near-UV (275 nm-390 nm) laser light. An interference pattern is generated at the core using a molded polymer phase mask with a square wave surface relief pattern. The light is directed through the phase mask, through a protective fiber coating, through the cladding, and into the core. The phase mask generates an interference pattern with a period half that of the surface relief pattern. Index of refraction changes occur at the bright fringes of the interference pattern--thus creating the grating. Advantages over existing mid-UV technology include lower fabrication costs for phase masks, simplified grating induction since fiber coatings do not need to be removed, and reduced infrared absorption caused by grating formation in the fiber.

This application is a divisional of U.S. patent application Ser. No.08/898,456 filed Jul. 24, 1997, entitled NEAR-ULTRA-VIOLET FORMATION OFREFRACTIVE-INDEX GRATING USING PHASE MASK, naming Dmitry S. Starodubovas sole inventor, which is a divisional of U.S. patent application Ser.No. 08/775,461, filed Dec. 30, 1996, entitled NEAR-ULTRA-VIOLETFORMATION OF REFRACTIVE-INDEX GRATING USING PHASE MASK, naming Dmitry S.Starodubov and Jack L. Feinberg as joint inventors, now U.S. Pat. No.5,745,617, issued Apr. 28, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to optical media and, more particularly,to the fabrication of gratings within optical media. A major objectiveof the invention is to provide for less absorptive refractive-indexgratings in optical fibers and to make such gratings easier and moreeconomical to fabricate.

Along with the increasing prominence of the Internet has comewideranging demand for increased communications capabilities, includingmore channels and greater bandwidth per channel. Optical media, such asoptical fibers, promise an economical alternative to electricalconductors for high-bandwidth long-distance communications. A typicaloptical fiber includes a silica core, a silica cladding, and aprotective coating. The index of refraction of the core is higher thanthe index of refraction of the cladding to promote internal reflectionof light propagating down the core.

Optical fibers can carry information encoded as optical pulses over longdistances. The advantages of optical media include vastly increased datarates, lower transmission losses, lower basic cost of materials, smallercable sizes, and almost complete immunity from stray electrical fields.Other applications for optical fibers include guiding light to awkwardplaces (as in surgery), image guiding for remote viewing, and sensing.

The signal carrying ability of optical fibers is due in part to thecapability of producing long longitudinally-uniform optical fibers.However, longitudinal variations in index of refraction, e.g., thoseassociated with refractive-index gratings, can be included in theoptical fibers to affect throughgoing pulses in useful ways. Gratingscan be grouped into short-period, e.g., about 0.5 micron (μm), orlong-period, e.g., about 200 μm, gratings. Short-period gratings canreflect incident light of a particular wavelength back on itself in thefiber. (Short-period gratings are also called Bragg gratings or Hillgratings.) Long-period gratings can couple incident light of aparticular wavelength into other co-propagating modes on the fiber. Someof these other co-propagating modes may be lossy, so the overall effectof the long-period grating can be to selectively block certainwavelengths from propagating efficiently through the fiber.

While there are many methods for establishing a refractive-index gratingwithin a fiber, the most practical methods involve exposingphotosensitive fibers to patterned light. The index of refraction ofcertain fiber-optic materials, such as germanium-doped silica, ischanged upon exposure to mid-ultra-violet (mid-UV) light, e.g.,wavelengths between 190 nanometers (nm) and 270 nm; thephoto-sensitivity of such a fiber can be enhanced by hydrogen loading.Lasers for altering the refractive index of fibers that span the abovemid-UV wavelength range include ArF excimer lasers with a laser outputat 193 nm and the fourth harmonic of a 1064 nm Nd:YAG laser at 266 nm.

It has been believed that the mid-UV light dislodges electrons atgermanium oxygen deficient centers (GODC) to cause the change in theindex of refraction. Exposing a germanium-doped fiber to mid-UV lightthat varies in intensity periodically in space creates a correspondingspatially varying pattern of refractive index in the fiber. Such aspatially varying index of refraction is referred to as arefractive-index grating.

Methods of generating the desired light pattern can be distinguishedaccording to whether or not they rely on interference. Methods notemploying interference rely on amplitude masks. For example, aphotoresist or metal amplitude mask can be photolithographically definedon the cladding of a waveguide or a fiber, the coating of which has beenremoved over the region where a grating is to be formed. However,diffraction effects limit the effectiveness of the amplitude mask,especially when applied to short-period gratings. In addition, the finestructure of the amplitude mask defining the dark regions can be burnedoff by absorbed laser energy.

Alternatively, a single slit can be stepped across the length of fiberin which the Bragg filter is to be defined. Such a method is disclosedin U.S. Pat. No. 5,105,209. The method has been extended recently towriting long period gratings using near-UV wavelengths using argonlasers with high-coherence. (E. M. Dianov, D. S. Starodubov, S. A.Vasiliev, A. A. Frolov, O. I. Medvedkov, Paper TuCC2, Vol. 1 of LEOS'96Annual Meeting Proceedings, pp. 374-375, 1996). Generally, the timerequired for the step-by-step writing is lengthy and the mechanicalprecision required for the stepping can be prohibitive.

More practical methods of inducing a Bragg grating take advantage ofinterference. A coherent laser beam can be split and the resulting beamcomponents can be made to intersect. Due to the wave nature of light,the intersecting components will add at some locations and cancel atother locations, creating a spatially alternating pattern of light anddark.

For example, U.S. Pat. No. 4,474,427 to Hill discloses a method in whichvisible light is launched into the core of a fiber and reflected at anopposite end of the core. The result is a standing wave with a periodcorresponding to half the wavelength of the light. Through aphotosensitive effect in the fiber, a refractive-index grating with thisperiod is written into the core of the fiber. In this case, the lightused was blue-green at around 480 nm. In this case, the gratings arecreated by two-photon absorption, corresponding to the energy associatedwith 240 nm light.

An important advantage of this core-launch approach is that neither thecladding nor the protective polymer coating needs to be removed for thegrating to be induced. However, this method is limited to producinggratings for reflecting wavelengths close to that of the writing light.Furthermore, the core-launch approach does not provide for gratings withan arbitrary spatial variation of index amplitude and period imposedover the length of the grating itself; these include chirped andapodized gratings.

More flexibility in defining gratings can be achieved by directinginterfering beams transversely. As disclosed in U.S. Pat. No. 4,807,950to Glenn et al., two beams directed transversely into a fiber can bemade to interfere. The spatially varying interference pattern creates aspatially varying refractive-index pattern. By changing the angle of thetwo incident light beams it is possible to vary the spatial period ofthe intensity pattern, which alters the reflecting wavelength of theresulting grating. The interference pattern of two light beams can becreated by the use of beamsplitters and mirrors, or with a prism by thetechnique of Lloyd's interferometer (U.S. Pat. No. 5,377,288).

To produce an interference pattern, a laser beam must be split and thenrecombined. Mirror vibrations and limits on coherence length can limitthe visibility of the interference pattern formed when the beamsrecombine. Addressing this problem, U.S. Pat. No. 5,367,588 to Hill etal. uses a phase mask in close proximity to a fiber to split the beamand direct its components so that they interfere at the fiber core. Thephase mask can be a block of material with a surface relief pattern thatacts as a series of beam splitters. Even low-coherence lasers, such asexcimer lasers, can be used with such a phase mask. So that it transmits245 nm light, the phase mask is formed of fused silica.

The fused silica is etched with an appropriate square-wave surfacerelief pattern using electron-beam lithography. When a phase mask isused, the period of the induced grating is one-half that of the surfacerelief pattern when the mask and core are parallel. The grating periodcan be increased slightly by tilting the mask slightly relative to thecore. (Larger tilts result in gratings that no longer reflect light intothe fiber.) Disadvantages of this phase-mask method include the cost ofthe phase mask: the main expense being the cutting of the pattern usingelectron-beam lithography. The cost scales with the length of the phasemask; long gratings are very expensive. It is not practical to fabricatelong gratings in this manner. Another expense is the fused silica whichmust be very pure to transmit mid-UV light.

There are also problems with obtaining an appropriate light. An excimerlaser can provide the mid-UV light, but has a short coherence length. Italso provides a pulsed rather than a preferred continuous output. Afrequency-doubled argon laser can be used for a continuous output, butfrequency doubling poses its own complications. Advances insemiconductor lasers promise better lasers in the desired frequencyrange, but these are not currently available. Thus, there remains roomfor more convenient and cost effective means for inducing gratings in anoptical fiber.

SUMMARY OF THE INVENTION

The present invention is directed to a method of making a phase maskusing radiation imagery and such a mask capable of being made by thismethod. The present invention enables inducing of gratings usinginterference of light having near-UV wavelengths in the range 275-390nm. This relaxes the constraint on the phase mask material; very purefused silica is not required. Lower purity fused silica, alternativeglasses, various polymers, and plastics can be used.

A refractive-index grating is to be formed in an optical waveguide suchas an optical fiber or a planar waveguide. At least at the time thegrating is to be formed, the waveguide must include material in whichchanges of refractive index can be photo-induced using near-UV light.The photo-sensitivity may or may not remain after the grating is formed.

The phase mask defines a spatially varying optical path lengththerethrough. The spatially varying optical path length varies either byvarying the physical path length or by varying the index of refractionof the phase mask or both. In either case, the spatial variation isalong the cross-section of a beam transmitted through said mask. As thephrase "through the mask" is used herein, it encompasses both the casewhere light enters a back face and exits a front face, and the casewhere the light enters the front face, is reflected off the back face,and exits the front face. In either case, the front face is taken to bethe face of the phase mask facing the fiber in which the grating is tobe induced.

Alternately, the front face of the phase mask can contain a surfacerelief pattern that is coated with a metal such as aluminum or adielectric layer to make it highly reflective. In this case light isreflected off of the front face of the mask to make an interferencepattern in the fiber core.

For example, U.S. Pat. No. 5,351,321 to Snitzer et al. uses a solidblock of material with a periodically varying index of refraction togenerate a grating; in this case, the plane is through the block ofsolid material. U.S. Pat. No. 5,367,588 to Hill et aL discloses a phasemask having a surface-relief pattern that creates a spatially varyingphysical path length. Note that the surface relief pattern defines aspatially varying refractive index over a plane through the surfacerelief pattern: the refractive index varies according to whether theplane intersects the protruding areas of mask material or the air orother material in the interstices. A third possibility is a phase maskwith a shield over a surface relief pattern. A fourth possibility is aphase mask with a reflective surface relief pattern. If the reflectivesurface relief pattern is arranged on its back face, a spatially varyingoptical path length is presented to light entering and exiting the frontface of the phase mask.

The present invention provides for molding a phase mask so that it has asurface-relief pattern suitable for generating a grating. A mold for thephase mask can include a substrate with an appropriate surface reliefpattern. The substrate surface relief pattern is technicallycomplementary to the surface relief pattern for the phase mask. However,where the surface relief pattern includes square or sine wave patterns,the surface relief patterns of the substrate and phase mask areessentially identical. In this case, the mold substrate can itself be aphase mask. Thus, existing phase masks for mid-UV light can be used toproduce phase masks for the near-UV light employed by the presentinvention.

The mold material begins as a liquid or plastic and ends as a solidstructure that is at least half as transmissive as it is absorptive oflight of the writing wavelength. The transmissivity of the structure isa function both of its material and the length of the light path throughthe material. Preferably, the transmissivity is much greater than theabsorption. The liquid can be a polymerizable liquid that solidifiesupon polymerization. Alternatively, the liquid can be anelevated-temperature melt, e.g., of plastic, that solidifies uponcooling. Also, the liquid can be a silica sol in a sol-gel process; itforms a gel that is dried and heated to form a solid glass. Aftersolidification, the resulting structure is removed from the substrate,yielding the desired phase mask. Optionally, the surface relief patterncan be coated with a reflective layer or layers to define a reflectivephase mask.

A method of the invention involves a submethod of fabricating a phasemask, as described above, and a submethod of using the phase mask togenerate a grating. The latter submethod involves disposing the phasemask in a suitable position near the optical fiber. Near-UV light istransmitted through or reflected from the phase mask to produce thedesired interference pattern through the fiber core.

The resulting fiber grating written with near-UV light, e.g., 334 nmproduced by an argon laser, can be differentiated from a fiber gratingwritten with mid-UV light, e.g., 245 nm. In the former case, theabsorption of light at 290 nm is at least ten times less than itsabsorption of light at 240 nm; in the latter case, the absorptions arewithin a factor of four of each other. In the former case, the fibergrating has a paramagnetic resonance spectrum in which the strength ofthe Ge(1) center is at least an order of magnitude less than thestrength of the GeE' center; in the latter case, the respectivestrengths are within a factor of three of each other.

What is surprising is that the grating-induction effectiveness of thenear UV light relative to the effectiveness of the shorter wavelengthlight is greater than would be expected by comparing the absorption ofan optical fiber at these wavelengths. A typical optical fiber is about1000 times more absorptive at 245 nm (mid-UV) than at 334 nm (near-UV).Yet the effectiveness of the grating induction at 245 nm is only 10times more than the effectiveness at 334 nm. The reason for thissurprising result is not completely understood.

By way of proposed explanation, and not of limitation, it is believedthat the electron loss previously assumed to contribute to the change ofrefractive index actually contributes more to an increase in absorption.A second mechanism, that of bond restructuring is more closely relatedto the change in refractive index. This bond restructuring is achievedmore directly using near-UV light than it is using mid-UV light.

On the other hand, near-UV light is less likely to cause electronlosses. Accordingly, the invention yields gratings that are lessabsorptive of throughgoing light than are fibers with gratings inducedby mid-UV light. When mid-UV light is used to induce gratings, theabsorption of the core region in which the grating is induced isincreased. This is especially true for fibers "loaded" with hydrogen toincrease their photosensitivity. When near-UV light is used, theincrease in absorption is much less, especially in the case of hydrogenloaded fibers.

Another major advantage of the invention is that the protective coatingof an optical fiber need not be removed for a grating to be induced. Thetypical silicone coatings are far more transmissive of at least somenear-UV wavelengths than of mid UV wavelengths. Therefore, thegrating-inducing laser light can be directed through the coating. Thisavoids the requirement of most grating induction methods, includingother transverse interference methods, of removing the coating to inducethe grating. Thus, the tedious steps of coating removal and replacementare avoided. This results in a considerable saving in manufacturing timeand cost.

Furthermore, a near-UV transparent fiber coating can serve as a phasemask by imposing on it a spatially varying refractive index. This can beaccomplished by heating a portion of the coating until it achievesplasticity. Then a surface-relief pattern mold can be pressed into thesoftened coating. The coating is cooled so that its surface assumes thesurface-relief pattern. The refractive-index varies in a plane throughthe surface-relief pattern between the refractive index of silicone andthe refractive index of air.

Since it has higher energy photons and is absorbed more completely,mid-UV light heats a fiber during writing more than does near-UV light.The heat so generated can be sufficient to erase the grating as it isbeing written. This problem is avoided at near-UV wavelengths which haveless energy per photon and are absorbed more weakly.

Another advantage of the present invention is that phase masks can bemuch more economical: molding can be much less expensive than etchingwith an electron stepper. In particular, long gratings can be much lessexpensive. Also, phase masks can be mass produced and standardized sincethey come from the same mold. In addition, less expensive materials canbe used for the phase mask. Moreover, it is easier to obtain lasers withthe desired characteristics at the longer laser wavelengths. These andother features and advantages of the invention are apparent from thedescription below with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a system for inducing a grating in anoptical fiber in a system employing a phase mask made in accordance withthe method of the present invention.

FIG. 2 is a schematic end view of the system of FIG. 1.

FIG. 3 is a flow chart of a method of making and using a phase mask inconjunction with the system of FIG. 1 in accordance with the presentinvention.

FIG. 4A-4C give a schematic representation of some of the steps of themethod of FIG. 3.

FIG. 5 is an alternative embodiment of the invention with a phase maskformed in the coating of an optical fiber.

FIGS. 6A and 6B are respectively schematic side and end views of a fiberand a reflective phase mask for inducing a grating in said fiber inaccordance with the present invention.

FIG. 7 is a schematic representation of state changes induced in a fiberusing near-UV light (solid lines) and mid-UV light (dashed lines).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a system AP1 for inducing inan optical fiber 10 an index-of-refraction grating 12 comprises acontinuous near-UV light source 20, a focusing lens 22, and a phase mask30, as shown in FIGS. 1 and 2. Optical fiber 10 comprises a core 14, acladding 16, and a coating 18. Core 14 has less than 4 micron (μm)diameter and is formed of (30 mol %) germanium-doped silica. (Typicalranges include 2-10 μm core diameter and 3-30 mol %.) Cladding 16 has adiameter of 125 μm and is formed of silica with an index of refractionslightly smaller than that of core 14 to promote internal reflections oflight transmitting through the core. Coating 18 is about 400 μm indiameter and is of a near-UV transmitting silicone. Core 14 can containboron and/or be hydrogen loaded to enhance photosensitivity of fiber 10to facilitate grating induction.

UV source 20 is an argon laser tuned and filtered to yield light 40 of asingle wavelength near 334 nm. Light 40 leaves source 20 as a collimatedbeam 42. Lens 22 redirects collimated beam 42 to define converging beam44 that focuses within or near core 14 so that that is the locus of amaximum interference effect.

Phase mask 30 is a silica-polymer-silica sandwich comprising apolymethylmethacrylate (PMMA) phase-mask body 32 with a surface reliefpattern 34, a glass shield 36 covering surface relief pattern 34, and aglass back support 38. PMMA is a polymer that is highly transmissive oflight at 334 nm. Surface relief pattern 32 acts as a bank of beamsplitters extending longitudinally parallel to fiber 10. Light 44originally directed transversely with respect to fiber 10 is split intorearwardly 46 and forwardly 46 directed beams that interfere with eachother at core 14. This interference at photosensitive core 14 results inthe formation of refraction-index grating 12. Glass shield 36 is 200 μmthick. A shield of less than 300 μm is preferred to limit losses ininterference strength. Back support 38 is 1 mm thick to provide rigidityto phase-mask body 32.

A method M1 for inducing grating 12 in fiber 10 in accordance with thepresent invention is flow charted in FIG. 3. Method MI includes twosub-methods: method M2 provides for fabrication of the phase mask, whilemethod M3 provides for use of phase mask in generating grating 12.

Method M2 begins at step S21 with obtaining, for example, by assembling,a mold form 40, as shown in FIG. 4A. Mold form 50 includes a substrate52 and four (two shown) sidewalls 54. Substrate 52 has a surface reliefpattern 56 inverse to that of to-be-formed phase mask 30. Generally, thesubstrate can be any of a variety of materials into which such a patterncan be formed, e.g., metal, glass, or a silica phase mask itself.

Inverse surface relief pattern 54 can be formed by a variety of methods,including the electron gun stepping used to form 245 nm phase masksdisclosed in U.S. Pat. No. 5,347,588. Conveniently, a phase mask madefor the purpose of writing with 245 nm light is used for substrate 52.In this case, the depth of the relief pattern is optimized for 245 nmwriting through a material with the index of refraction of silica, e.g.,about 1.5. If the same mask were used for writing at 334 nm, theinterference pattern would not be as strong. However, this loss ofinterference-pattern visibility can be compensated, at least in part, byusing a phase-mask body material with a refractive index higher thanthat of the 245 nm mask used as a mold substrate. More generally, if thegrating strength need not be maximal, the use of existing 245 nm (orother mid-UV) phase masks for molding near-UV phase masks can beconvenient and economical.

Step S22 involves conforming moldable material 58 to sidewalls 54 andsurface relief pattern 56. In the preferred embodiment, PMMA pre-polymeris poured into mold form 50, as shown in FIG. 4A, so that it conforms tosurface relief pattern 56 and is confined laterally by sidewalls 54, asshown in FIG. 4B. Support 38 is used as a mold cover to ensure a flatback surface of phase-mask body 32. Alternatively, a separate mold coverplate can be used and replaced by a support, if desired, aftersolidification. In this latter case, the mold cover plate need not betransparent to near-UV light.

Step S23 involves solidifying the moldable material. In the presentcase, solidification occurs through polymerization. Alternatively, themoldable material can be a fluid or plastic at an elevated temperaturethat solidifies upon cooling to room temperature. Herein, words with"solid" at the root refer to materials that can maintain a surfacerelief pattern--including glass in its supercooled state. Thus, themoldable material can be sol-gel glass, in which case the phase mask issupercooled glass.

Step S24 involves separating the phase mask from the mold form. This canbe accomplished by removing four sidewalls 54 and prying loose phasemask 30 from substrate 52. Shield 36 can be applied over surface reliefpattern 34 as indicated in FIG. 4C. Shield 36 prevents the intersticesof surface relief pattern 34 from being filled with particles that canbe dislodged from a fiber coating due to heat as a grating is beingwritten. Alternatively, the interstices can be pre-filled with anothermaterial having a refractive index different from that of PMMA. Anotheralternative is to allow the shield to enclose the surface relief patternand fill the interstices with a fluid (gas or liquid) with a suitablydifferent refractive index.

Method M3 begins with a step 31 of arranging the grating inducingequipment as shown in FIGS. 1 and 2. Phase mask 30 can be placed oncoating 18 of fiber 10 over the core region in which grating 12 is to beformed. If a stronger grating is desired, coating 18 can be removedlocally and phase mask 20 disposed against or near cladding 16. Thepositioning and orientation of phase mask 30 are such that when laserlight is transmitted therethrough, an interference pattern is formedwhich extends through core 14. Note that the period of the grating canbe adjusted by tilting phase mask 20 relative to the longitudinal extentof fiber 10.

In addition, step 31 involves arranging laser 20 and focusing lens 22,as indicated in FIG. 1, so that light is directed through phase mask 30and into core 14. Optionally, fiber 10 and phase mask 30 can be mountedon a stage to provide for movement together relative to the writinglight. Such movement allows writing gratings that are longer than thelongitudinal extent of the interference pattern.

Step 32 involves transmitting laser light through phase mask 30 togenerate an interference pattern in core 14. If a long grating isdesired, the stage can be moved during exposure. Each area in which thegrating is to be formed is continuously exposed for about one minute ata spatially averaged intensity of about 5 kilowatts per squarecentimeter.

In accordance with a variation of the present invention, an opticalfiber 80 includes a core 82, a cladding 84, and a coating 86, as shownin FIG. 5. Formed in coating 86 is a square wave surface relief pattern88. Coating 86 thus can serve as a phase mask for generating aninterference grating 90 in core 82. Surface relief pattern 88 can beformed for example by heating coating 86 until it is tacky and pressinga surface relief pattern, e.g., surface relief pattern 56 of substrate52, into coating 86. Coating 86 is then cooled and the mold surfacerelief pattern removed.

In the case of fiber 80, no separate phase mask is required to inducethe grating. Another advantage of fiber 80 is that the phase mask ispermanently aligned with the grating. If the grating weakens (due toexposure to heat), it can be reestablished by shining near-UV lightthrough pattern 88 into core 82.

In accordance with a further embodiment of the invention, shown in FIGS.6A and 6B, a phase mask 100 has a back-face surface relief pattern 102on its back face. Surface relief pattern 102 is coated with aluminummetal to enhance its reflectivity to near UV light. Incidentnear-UV.light 104 is directed into the front face of mask 100, isreflected off reflective surface relief pattern 102 to form reflectedbeams 106. Reflected beams 106 exit the front face of phase mask 100 andinterfere at fiber 110 to create a spatially-varying light pattern.Alternately, the mask can have a surface relief pattern on its frontface (not shown) and coated with metal to be reflective, and incidentnear-UV light 104 is directed into the front face of such mask and isreflected off its reflective surface relief pattern to form reflectedbeams 106. The light pattern from these reflected beams 106 creates arefractive-index grating in the core 114 of fiber 110. One advantage ofusing a phase mask with a reflective surface relief pattern on its backface is that, since the surface relief pattern is on the back (away fromthe fiber) face of the mask 100, the depth of the surface relief patterncan be made shallower by a factor of 2n/(n-1)≈6 (compared to the depthof the surface relief pattern when using the transmissive geometry ofFIG. 1).

As indicated above, the present invention is surprisingly effective atgenerating gratings given the much lower absorption at the longerultraviolet wavelengths. By way of explanation and not of limitation,the following theoretical discussion may aid understanding. FIG. 7presents various states of a GODC in an optical fiber. The ground stateis shown at A. A photon of 245 nm light can excite the site to a singletstate B. Either spontaneously or due to the energy of a second 245 nmphoton, an electron can be released, corresponding to state C. It hasbeen believed that it was necessary to dislodge an electron from a GODCin a germanium doped optical fiber to produce a change of refractiveindex. As explained below, this belief may have been erroneous.

Also spontaneously, a singlet state B can drop to a triplet state D.Another 245 nm photon can cause an electron release to state E.Otherwise, it is believed that the triplet state can drop to a state Fthough a bond breaking. The bond breaking results in a change of bondstructure. It is proposed that it is this bond restructuring rather thanthe loss of electrons that contributes to the change in the index ofrefraction. Instead, the electron loss is associated primarily with anundesirable increase in a fibers absorption of throughgoing light.

When 334 nm light is used for writing, a single photon is generally notenergetic enough to reach the singlet state B. Instead, triplet state Dis entered directly. Again, continuous wave 334 nm light is generallynot efficient in exciting from triplet state D to electron loss state E.Instead, triplet state D spontaneously converts to bond-modified stateF. Thus, the conversion to bond breaking and molecular reorientation isrelatively direct and occurs relatively frequently. The decrease inabsorption due to the exclusion of the ground to singlet path iscompensated by the increased efficiency of the index-changing process.This explains the surprising effectiveness of 334 nm light in generatingchanges in index of refraction.

Concomitantly, the 334 nm light causes a lower loss of electrons. Thus,there is less of an increase in absorption in the fiber so that fibertransmission characteristics in the infrared wavelength transmissionwindows at 1.3 microns and 1.5 microns are better preserved when 334 nmlight is used. 245 nm light, on the other hand, undesirably increasesabsorption in the infrared, especially in a hydrogen-loaded fiber. Theincrease in infrared absorption can be ameliorated, for example, byannealing the fiber for approximately two days at approximately 150° C.;the present invention obviates the need for this time-consumingannealing.

Fibers made in accordance with method M1 are distinguishable from fibersmade using shorter wavelength ultraviolet light. In general, there isless damage and a smaller increase in absorption. In the former case,the absorption of light at 290 nm is at least ten times less than itsabsorption of light at 240 nm; in the latter case, the absorptions areusually within a factor of four of each other. In the former case, thefiber has a paramagnetic resonance spectrum in which the Ge(1) center isnot seen; the strength of the Ge(1) center is at least an order ofmagnitude less that the strength of the GeE' center; in the latter case,the strengths are within a factor of three of each other.

The present invention applies not only to single-core fibers, but alsoto multi-core fibers, such as those used for fiber lasers. In addition,the gratings can be induced in both single-mode and multi-mode fibers.Furthermore, the invention applies to optical media other thanfibers--for example, planar optical waveguides on a substrate.

The present invention allows a selection of light sources to provide the275 nm to 390 nm near-UV light. Argon lasers, nitrogen lasers (337 nm),helium-cadmium lasers (325 nm and 354 nm), excimer lasers, e.g.,xenon-chloride (308 nm), are all known to produce light in the requiredwavelength range. Argon lasers can provide light over a range extendingfrom 275 nm to 386 nm. Krypton lasers span the range 337 nm to 356 nm.Furthermore, various infra-red lasers can provide harmonics in thedesired near ultra-violet range. Advances in laser technology, includingdye-lasers and diode-pumped solid-state lasers, promise to provide morechoices in writing light sources.

The periodicity of the phase mask can be in the form of a surface reliefpattern. In this case, the surface relief pattern defines a spatiallyvarying index of refraction at the mask-air interface. Alternatively,the phase mask can include an internal spatially varying index ofrefraction and not require a surface relief pattern. In either case, thegrating period can be constant or varying, e.g., chirped, to generate acorresponding periodicity in the fiber grating. The phase mask caninclude an amplitude gradient or an amplitude filter which can be usedwith the phase mask to create intensity variations along the grating.The phase-mask material can be a polymer, a plastic, a silica glass, asilicate glass (e.g., one containing fluorine or phosphorous), or othernear-UV transmissive material.

As with gratings written with 245 nm light, the grating period isone-half that of a phase mask when 334 nm light is used and the mask isparallel to the core, i.e., the grooves of the mask are perpendicular tothe fiber axis. The grating period can be modified by tilting the phasemask relative to fiber core. However, if the angle is too large, theresulting "blazed" grating will deflect light out of the core ratherreflect it down a return path. Also, the angle of the grating in thecore can be adjusted by arranging the surface relief pattern of thephase mask at an angle to the core. In the case of the surface reliefpattern, it can be a square wave, a sine wave, or any number of shapes.The phase mask can include curved surfaces to serve as a focusing lens.

Much of the preceding discussion has focused on the flexibility thepresent invention affords in selecting a material for the phase mask. Asimilar flexibility is afforded to the selection of the material forfocusing lens 22 since this lens does not need to be transparent to 245nm light. In particular, a lens or combination of lenses can be moldedplastic instead of ground fused silica. Molding allows a larger range oflens geometries, in particular aspherical geometries. This in turn,allows greater optimization of the optical path used for writing agrating.

In the preferred embodiment, the coating is left intact during writing.However, a stronger grating may be obtainable by removing the coating,so this option is within the scope of the invention. In most cases,however, the convenience of leaving the coating in place will outweighgains in grating strength. While the disclosed fiber is hydrogen loadedand/or highly doped with germanium, other light-sensitive fibermaterials can be used. These and other variations upon and modificationto the disclosed embodiments are provided for by the present invention,the scope of which is limited only by the following claims.

What is claimed is:
 1. An optical fiber comprising:a cladding; a corehaving a core segment with a refractive-index grating having an averageperiodicity between 0.2 μm and 2.0 μm, said core segment having anabsorption of light at 290 nm is at least 10x less than its absorptionof light at 240 nm.
 2. An optical fiber as recited in claim 1 whereinsaid core segment has a paramagnetic resonance spectrum in which thestrength of the Ge(1) center is at least an order of magnitude less thanthe strength of the GeE' center.
 3. An optical fiber comprising:acladding; a core having a core segment with a refractive-index gratinghaving an average periodicity between 0.2 μm and 2.0 μm, said coresegment has a paramagnetic resonance spectrum in which the strength ofthe Ge(1) center is at least an order of magnitude less than thestrength of the GeE' center.